1. Field of the Invention
The present invention is directed to a device for determining the rotational number of a dc motor which is used particularly for driving a moving body of an automobile accessory such as a sunroof, a pane of glass in a power window, or an electrically operated seat device.
2. Description of Related Art
A conventional device of the kind is disclosed in the U.S. Pat. No. 5,497,326 granted to Berland et al. In this patent, the device issues a signal indicative of the amount of motor rotation. The signal is in the form of a pulse signal, which is generated in such a manner that a ripple component of a current flowing through the motor is formed into a pulse mode by being passed through a circuit including a low-pass filter and other components.
The frequency range of the ripple component of the current which the low-pass filter can treat is limited, which necessitates the provision of a compensating means to make the signal correct even though the range of the frequency range of the ripple component falls outside the ability of the low-pass filter.
However, providing such a compensating means is very cumbersome.